The decision comes as the crisis at the EU's eastern border pitting Ukraine against Russia poses one of the biggest foreign policy challenges for the bloc in decades.

Ms Mogherini, a 41-year-old centre-left politician, has been Italy's foreign minister only since February, drawing criticism that she lacks experience.

The EU leaders also elected Polish Prime Minister Donald Tusk to succeed European Council President Mr van Rompuy in December.

Ms Mogherini "will be the new face of the European Union in our day-to-day dealings with our partners in the world," Mr van Rompuy said.

The highly visible job as EU foreign policy chief entails flying across the world and meeting the great and powerful to deal with anything from the fighting in eastern Ukraine to the crises in the Middle East.

However, the EU's top diplomat often has had little leeway because the bloc's member nations jealously guard foreign policy as a national matter, leaving the foreign policy chief the role to hammer out compromise positions.

Ms Mogherini said she will work relentlessly to promote European projects on the international stage, while fighting off crises or seeds of discord that could undermine the bloc's success.

"We are a dream come true, having to be careful that the dream doesn't turn into a nightmare," she said.